The reproductive and parental behaviors of monogamous primates have received little study in comparison with polygamous or promiscuous species. However, several features of the social biology of monogamous primates suggest closer parallels to human social organization than other primates. The Callitrichidae (marmosets and tamarins) are monogamous species which show the following: (1) little sexual dimorphism either behavioral or physical, (2) infrequent courtship except in the initial stages of pair formation, (3) delayed sexual maturation of offspring when living with parents, (4) older siblings assist in the rearing of offspring, (5) adult male is actively involved in parental care. We propose to study each of these phenomena in the cotton-top tamarin (Saguinus oedipus oedipus). First, we have developed techniques for measuring urinary estrogens, progestins and LH in females so that we can describe their hormonal cycles. We will be especially interested in the hormonal cycles of immediately postpartum females which show a two month infertility period, of young females while living in their family groups and after being mated with a male, of females which experience delays in conception. Second, we have found a subtle behavioral cue (anogenital scent marking) which appears to correlate with sexual activity. We will correlate this and other behaviors with the hormonal cycle to determine the behavioral and physiological correlates of proceptivity, attractivity, and receptivity. Third, we will study the development of social behavior in order to determine what specific social experience young tamarins must have with infants in order to become competent parents. We will also try to determine how the suppression of sexual activity in group living offspring is maintained and what experiential variables affect the development of parental care. These studies should give us a definitive description of both hormonal and behavioral aspects of reproductive and parental behavior in a monogamous primate, and should give us a better understanding of human reproductive and parental behavior.